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The product of inertia of a body with density function  
\[\rho (x,y,z)\]
  about the
\[xy\]
  plane is  
\[I_{xy} = \int_z \int_y \int_x xy \rho (x,y,z) dx dy dz\]

\[xz\]
  plane is  
\[I_{xz} = \int_z \int_y \int_x xz \rho (x,y,z) dx dy dz\]

\[yz\]
  plane is  
\[I_{yz} = \int_z \int_y \int_x yz \rho (x,y,z) dx dy dz\]

Example: If  
\[\rho (x,y,z) =x+yz\]
  over the region bounded by  
\[0 \leq x \leq 1 , 0 \leq y \leq 2 , 1 \leq z \leq 3\]
  then
\[\begin{equation} \begin{aligned} I_{xy} &= \int^3_1 \int^2_0 \int^1_0 xy (x+yz) dx dy dz \\ &= \int^3_1 \int^2_0 \int^1_0 x^2y +xy^2 z dx dy dz \\ &= \int^3_1 \int^2_0 [ \frac{x^3y}{3} + \frac{x^2y^2z}{2}]^1_0 dy dz \\ &= \int^3_1 \int^2_0 \frac{y}{3} + \frac{y^2z}{2} dy dz \\ &= \int^3_1 [\frac{y^2}{6} + \frac{y^3 z}{6} ]^2_0 dz \\ &= \int^3_1 \frac{2}{3} + \frac{4z}{3} dz \\ &= [\frac{2z}{3} + \frac{2z^2}{3}]^3_1 \\ &= (\frac{2 \times 3}{3} + \frac{2 \times 3^2}{3} ) - (\frac{2 \times 1}{3} + \frac{2 \times 1^2 }{3}) = \frac{20}{3} \end{aligned} \end{equation}\]

The product of inertia has a physical meaning.
\[I_xy = I_yx\]
  is the inertia of a mass rotating around the  
\[x\]
  axis against its rotation around the  
\[y\]
  axis.